Add Civ5 Heavy Charge fall-back uniques

[Fanfblrik]

Summary

• Engine support for RekMOD Heavy Charge (Civ5-style melee push-back + trapped Strength).
• New unique Forces the enemy to fall back when dealing more melee damage.
• New conditional when the enemy cannot fall back for +50% Strength when the defender has nowhere to retreat.
• Shared MeleeRetreat with Slinger withdraw (escort-safe; keeps carried units; canMoveTo destinations).

Test plan

• BattleTest heavy charge tests (push + trapped +50%)
• Manual: unit with both uniques attacks — defender falls back when damaged more and a tile is free
• Manual: surrounded defender — no fall-back, attacker gets +50% Strength

[yairm210]

This looks like the reverse of the slinger retreat mechanic, they should share logic

[Fanfblrik]

Addressed review:

• Extracted shared MeleeRetreat used by both Slinger-style withdraw and Heavy Charge push (same candidate rings + free teleport).
• Escort/guard: blocked (same as withdraw) so we do not abandon a partner.
• Carried units: teleport now keeps payloads like other UnitMovement teleports.
• Destination checks: only canMoveTo (removed redundant land/city filters).

Heavy Charge still picks the furthest valid tile instead of RNG; withdraw keeps seeded random among preferred/secondary tiles.